Method and apparatus for adjusting operation of wire section

ABSTRACT

Adjusting the operation of a wire section, in which the development of the consistency of stock on the wire section ( 2 ) is determined and the effect of the consistency determined above on the formation and/or porosity of a paper web ( 3 ) is determined. The consistency developed on the wire section is adjusted based on a quality property of paper and/or by optimising a cost function, the quality property of paper comprising the formation and/or the porosity and/or a combination defined by means of the formation and/or the porosity and the cost function including the effect of at least the formation and/or the porosity.

The invention relates to a method for adjusting the operation of a wiresection.

The invention also relates to an apparatus for adjusting the operationof the wire section.

A paper machine includes a headbox, from which stock is fed onto thewire section, or former, where the stock forms a fibre web or sheet.From the wire section, the fibre web is conveyed to a press section andfrom there to a drying section, and thereafter the paper is reeled usinga reel.

The object of controlling the operation of a wire section is to provideon the wire section a web that is as homogeneous as possible. Thehomogeneity may be evaluated as a formation that describes the basisweight small-scale distribution at the web level. The structure of theweb in z direction must also be appropriate regarding the printingproperties. The formation affects the physical and optical properties ofthe paper. As the formation becomes worse, the occurrence probability ofthe thin spots increases, whereby the number of pinholes increases andthe unevenness of the strength properties increases. Manufacturingcoated types of paper in particular, the increased air permeability ofthe paper web, or the porosity, and the possible pinholes deterioratethe runnability of a coater. When calendering a web provided with a poorformation, the density distribution in the paper level direction becomesuneven, and causes the printing ink to be unevenly absorbed and thus apatchy print quality. The fines distribution in z direction of the paperweb is also significant when the printing properties of the finishedpaper are evaluated. Particularly the difference in the amount of fineson the upper and lower side of the paper web should generally be assmall as possible. In addition, the dry solids content of the paper websucceeding the wire section is a significant factor for the operation ofthe entire paper machine. If the dry solids content is too low, the edgecutting deteriorates, which increases edge faults. Likewise, too low drysolids content weakens the reliability of the operation, when the paperweb is conveyed from the wire section to the press section. Furthermore,when the dry solids content is loo low, there is a chance for sheetcrushing, for instance, when the paper is pressed, the difference indraw between the press and dryer section must be increased, and aboveall the runnability of the paper machine declines.

Typically, the wire section is regulated by adjusting the vacuums ofdewatering elements in the wire section and the slice opening of aheadbox. Adjusting the slice opening may affect the consistency ofheadbox stock and thus increase or reduce the amount of water to beconveyed onto the wire section. It is also possible to adjust thetensions of the wires in the wire section and thus to affect thedrainage of the dewatering elements.

An operator of a machine typically controls the operation of a wiresection, and builds his/her evaluation on the operation of the processon a visual estimation on the formation of manufactured paper, andsometimes on the probable formation measurements to be carried out. Insome cases, the drainage state is observed using drainage measurementsthat allow the operator to empirically estimate the quality of the webto be formed and if necessary to adjust the vacuums of the dewateringelements or the slice opening of the headbox. A cross web sample, fromwhich the paper web properties such as the formation are evaluated, isalways taken when a roll is changed, i.e. typically about once per hour.

What significantly affect the drainage taking place on the wire sectionare the infiltration properties of the stock, which strongly depend onthe freeness, the chemical state, the fibres clinging to one anotheri.e. flocculation, the fines content and temperature of the stock. Theinfiltration properties of the stock cannot currently be measured inconditions that fully correspond with those of a paper machine, andtherefore the behaviour of the stock in the drainage process on the wiresection cannot be fully predicted in advance. In addition, due to thevarying stock properties required by different paper grades, thebehaviour of the drainage process may change several times during theday in the paper machine. Moreover, difficulties may occur in the stockpreparation process owing to the changes taking place in the mutualdosage ratios of the stock components that may be caused by the changesin the dosage of the broke. It may therefore be very difficult for theoperator to optimally manage the control of the wire section. Since theformation of the bottom of the paper cannot be assessed other than usingrarely taken samples, the operator must leave an extensive safety marginfor the control in order to avoid sheet crushing, when the sheet isaffected by the dewatering elements loading the web, or the web frombecoming flocked owing to the possible changes occurring in theinfiltration properties of the stock taking place between sampling.

FI publication 97 244 shows a method for adjusting the drainage of thewire section, in which method each dewatering unit is provided with adrainage strategy. The amount of water drained by each dewatering unitis measured and the measured amount of water is compared with a setvalue. If the removed amount of water and the set value are of differentsize, the rotation speed of the electric motor of the vacuum pump in thedewatering unit is adjusted, for instance. U.S. Pat. No. 5,879,513discloses a method for adjusting drainage so as to measure the amount ofwater to be drained in the dewatering element and to adjust the vacuumcapacity on the basis thereof. However, in both above publications theoperator has to determine the set values in the conventional way shownabove. Consequently, all the problems are present in the solution foroptimising and adjusting the wire section according to the citedpublications.

Article “Effect of Vacuum Level and Suction Time on Vacuum AssistedDrainage of a Paper Machine Wire Section”, Kari Räisänen, HannuPaulapuro and Ari Maijala, APITA 48^(th) Annual General Conference,Melbourne 1994 studies how the vacuum in the dewatering elements of awire section affect the formation of a web. The article also presents amethod, which tends to predict the drainage in a wire section utilizinga mathematical formula. The idea is to be able to minimize the energyconsumption of a wire section or to achieve a dry solids content in theweb that is as extensive as possible using particular energyconsumption. The solution does not adequately consider the rapid changesoccurring in the stock or in the process, and said solution is thereforenot able to optimise the operation of the wire section as accurately asdesired.

FI publication 19992430 shows a method for determining the dry solidscontent of a paper machine and the development and/or control thereof.In this method, the dry solids content is determined at a desired pointbased on a measured liquid amount and a dry basis weight of the web.Measuring the amount of drainage in the dewatering elements of the wiresection and taking the determined dry solids content into account, thedry solids content of the paper web can be determined on the desiredlocations. The obtained dry stuff information allows adjusting, forinstance, the pressing pressures and/or the vacuum levels in the wiresection in order to achieve the desired dry stuff development. However,the solution does not allow optimising the operation of the wire sectionas accurately as desired.

EP publication 1 063 348 discloses a method for controlling the amountof water on the paper web and for preventing flocks to be formed on thepaper web to be produced. In this method, the amount of water on thepaper web is determined in the direction of movement of the web ondifferent locations of the wire section, wherefore the amount of waterto be removed from the web is adjusted by controlling the function ofsuction boxes. In addition, after the press section or the dryersection, the light transparency characteristic of the web is measured,on which basis the operation of blades arranged on the wire section iscontrolled, the blades being intended to cause turbulence breaking fibreflocks. The control is thus based on controlling the angle of themechanical components in the wire section. However, such a solution isnot capable of optimising the operation of the wire section as desired.

U.S. Pat. No. 5,825,653 shows an adjustment method for a wire sectionbased on a fluid flow model, where flow calculation allows adjusting thewire section. A physical fluid flow model is formed in the solution thatis based on the drainage of the wire and the flow state of the stocksuspension. The drainage of the wire is measured from several locationson the wire section by measuring the amount of water drained fromdifferent locations, and the flow state of the stock suspension isdetermined by means of the stock jet velocity, the wire speed and theconsistency of the stock. The quality of the paper is monitored at thedry end of the paper machine. The model determines the aimed flow stateas well as the difference between the aimed flow state and the currentflow state, from which a cost function is formed that allows determiningnew control and set values to reach the aimed flow state. The solutionthus requires forming a physical fluid flow model, whereby the methodbecomes very complicated and requires considerably more expertise.

It is an object of the invention to provide a method and an apparatus toallow controlling the operation of a wire section optimally.

The method of the invention is characterized by determining thedevelopment of the consistency of stock on a wire section, determiningthe effect of the consistency determined above on the formation and/orporosity of a paper web and adjusting the consistency developing on thewire section based on a quality property of paper and/or by optimising acost function, the quality property of paper comprising the formationand/or the porosity and/or a combination defined by means of theformation and/or the porosity, and the cost function including at leastthe effect of the formation and/or the porosity.

The apparatus of the invention is characterized by comprising means fordetermining the development of the consistency of stock on a wiresection, means for determining the effect of the consistency determinedabove on the formation and/or porosity of a paper web and means foradjusting the consistency developing on the wire section based on aquality property of paper and/or by optimising a cost function, thequality property of paper comprising the formation and/or the porosityand/or a combination defined by means of the formation and/or theporosity, and the cost function including at least the effect of theformation and/or the porosity.

The essential idea of the invention is that the development of theconsistency of the stock on a wire section is determined and the effectof the consistency on the formation and/or porosity of the web isdetermined. Furthermore, the consistency arriving at a particulardewatering element is adjusted based on said quality property of paper,i.e. the formation and/or porosity, and/or by optimising a costfunction, which contains the effect of the form ation and/or theporosity. The idea of a first embodiment is to determine an optimalvalue for the consistency in such a manner that the ratio between afirst derivative of said quality property of paper and/or the costfunction and a first derivative of the consistency is determined. Theidea of a second embodiment is to determine a correct control directionof the consistency using the ratio between the first derivative of thequality property of paper and/or the cost function and the firstderivative of the consistency, as well as the magnitude of a controlstep using the ratio between a second derivative of said qualityproperty of paper and/or the cost function and the first derivative ofthe consistency. The idea of a third embodiment is to determine thedevelopment of consistency on the wire section by determining the drysolids content on the web succeeding the wire section and measuring thewater amounts drained by the dewatering elements in the wire section andcalculating the development of the consistency on the wire section onthe basis of the drained water amounts. The idea of a fourth embodimentis to adjust the consistency by adjusting a slice opening of a headboxand/or the drainage of the dewatering elements on the wire section. Theidea of a fifth embodiment is to employ a fuzzy controller to implementthe adjustment of the consistency, in which the adjustable consistencyand the adjusting variables are provided with boundary conditions. Theidea of a sixth embodiment is to adjust the consistency by optimising inaccordance with such a cost function that includes both the qualitydeviation cost and the control cost. Thus, the quality deviation costtakes into account the runnability of the process and/or the dry stuffrequirement set by the quality property of paper between differentelements and/or the wire section and the press section. Then again, thecontrol cost considers the required power and/or driving output of thewire section in order to achieve drainage.

The invention provides such an advantage that the operation of the wiresection can be adjusted as optimally as possible to provide a web thatis excellent regarding the formation, porosity and dry solids content.In the embodiment utilizing the cost function, the provided costfunction allows determining the weighting of different qualityparameters concerning the adjustment measures aiming at a mostpreferable operation point. It is possible to select whether the mainemphasis lies on formation or porosity. The invention enables tooptimally adjust the consistency arriving at each dewatering element insuch a manner that the safety margin concerning the adjustment may bevery small. Furthermore, the invention allows controlling thedevelopment of the consistency of the web particularly well.

In this application, the term ‘paper’ refers in addition to paper alsoto board and tissue.

The invention will be explained in more detail in the accompanyingdrawings, in which

FIG. 1 is a side view schematically showing a paper machine;

FIGS. 2 and 3 are block diagrams showing the control system of a wiresection; and

FIG. 4 is a block diagram showing how to optimise the operation of awire section.

FIG. 1 schematically shows a paper machine. The paper machine comprisesa headbox 1, from which stock is supplied to a wire section 2, i.e. aformer, where a paper web 3 is formed of the stock. The paper web 3 isconveyed to a press unit 4 and from there to a dryer unit 5. From thedryer unit 5 the web is conveyed to a reel 6. A paper machine may alsocomprise other parts, such as a size press or a calender, which are notshown in FIG. 1 for the sake of clarity. Furthermore, the operation of apaper machine is known to those skilled in the art and is therefore notexplained in detail in this context.

In FIG. 1, a so-called gap former forms the wire section 2. The gapformer comprises an inner wire 7 and an outer wire 8, and a jet of theheadbox 1 is driven into the gap between said wires. The wires 7 and 8are guided by guide rolls 9. Some of the rolls are made movable in sucha manner that the tensions of the wires 7 and 8 can be adjusted bymoving said strecher rolls 17. In FIG. 1, a double-ended arrow indicatesthe movable strecher rolls 17.

Water is at first removed from the stock on the wire section 2 using aforming roll 10. The drainage of the forming roll 10 is typically 65 to80% of the flow of the headbox 1. What affects the drainage pressure ofthe forming roll 10 is the pressure caused by the tension of the innerwire 7, the pressure reduction towards the outer wire 8 caused bycentrifugal acceleration and the pressure prevailing inside the formingroll 10. Water is removed from the stock in the area close to theforming roll 10 both towards the forming roll 10 and on the side of theinner wire 7. Because of a continuous and symmetrical drainage, thetight surface layers of paper holding the fillers and fines on the paperare infiltrated. Water is removed without pulsating heavily in bothdirections simultaneously making the surfaces of the web more compact bybringing fines thereto.

After the forming roll 10, there is provided a multifoil shoe 11, whichis composed of several blades. The radius of curvature in the multifoilshoe causes a drainage pressure. A trailing angle is formed over anindividual blade within the area of the multifoil shoe 11 that is causedby the geometry of the blade. Because of the blades, the drainage of themultifoil shoe 11 becomes pulsating. The pressure pulses caused by themultifoil shoe and by other possible loadable blades 18 create shearforces into the free suspension within the web that further createvelocity gradients breaking the fibre flocks in the suspension. Breakingthe fibre flocks improves the formation of the web. The loadable blades18 are not required in all former types. If the former is provided withloadable blades 18, said blades enable to increase the strength of thepulsation within the area of the multifoil shoe 11. The multifoil shoe11 typically comprises several low-pressure chambers, whereby the use ofvacuum strengthens the pressure pulses and increases the drainagepressure caused by the radius of curvature of the element. Increasingthe vacuums of the multifoil shoe 11 generally also makes the webcompact. Adjusting the drainage of the forming roll 10 and the multifoilshoe 11 enables to affect the quality property of paper, such asformation, porosity and the form of the filler distribution. Thedrainage of the forming roll 10 indirectly affects the formation,porosity and bonding strength of the paper. If the drainage of theforming roll 10 is increased too much, the input consistency of themultifoil shoe is reduced too much and the formation of the paperdeteriorates. At the same time, the web 3 becomes more compact and thebonding strength improves.

The consistency of the stock conveyed onto the wire section 2 may rangebetween 0.2 and 1.2%. The dry solids content of the paper web 3 leavingthe wire section 2, in turn, typically ranges between 15 and 20%. Theforming roll 10 is used to remove about 65 to 80% water from the flow ofthe headbox 1, in which case the consistency of the stock ranges from 2to 3%. After the multifoil shoe 11, the consistency is approximately 4to 5%. After the multifoil shoe 11, the wire section 2 also comprises acouch roll 12 and one or more suction boxes 13 as dewatering elements.In addition, one or more suction boxes can be provided before the couchroll 12. The couch roll 12 and the suction boxes 13 may affect thequality property of paper only slightly. However, the dewateringelements enable to increase the dry solids content of the paper web.

The paper machine also comprises measuring instruments for measuring theproperties of the paper web 3. A first measuring instrument 14 may bearranged for instance in connection with the press section 4 and asecond measuring instrument 15 may be arranged after the drying section5. The paper machine also comprises a control unit 16, to which dataconcerning the properties of the paper web 3 are supplied from themeasuring instruments 14 and 15. In addition, measuring data from theprocessing elements of the paper machine is supplied to the control unit16. The control unit 16 controls, for instance, the dewatering elementsof the wire section 2, such as the forming roll 10, the multifoil shoe11, the couch roll 12 and the suction boxes 13, particularly the vacuumsthereof. The control unit 16 also controls the tension of the wires 7and 8 by controlling the movable strecher rolls 17. Furthermore, thecontrol unit 16 adjusts a slice opening 1 a of the headbox 1.

FIG. 2 is a block diagram showing the control system of a wire section.The control unit 16 is a multivariable controller, which provides localcontrollers with set values. The multivariable controller includes adynamic model describing the operation of the wire section that isupdated in accordance with the measurements. Local controllers adjust,for example, the vacuums of the dewatering elements in the wire section2 or the tensions of the wires 7 and 8 or the slice opening 1 a of theheadbox. The aim of the multi-variable controller is to stabilize thequality property of paper by changing the running parameters of the wiresection 2 in such a manner that the effect of possible interferencetaking place during the infiltration of the stock on the operation ofthe wire section 2 can be prevented, and to adjust the consistency ofthe stock for the elements in the wire section affecting the quality ofthe paper so that the formation and/or porosity of paper can be as goodas possible.

The local controllers thus affect the process that provides paper. Thelocal controllers are not explained in more detail in this context,since the structure and function of said controllers are apparent forthose skilled in the art.

The feedback required in adjustment is obtained firstly by measuring thequality property of paper. The measuring instruments 14 and 15 may beused to measure, for instance, the basis weight, moisture, on lineformation and porosity of the paper. Two-sidedness can be estimatedusing laboratory measurements. What are measured from the process arefor instance vacuums, flows and consistencies. Measurable propertiesinclude the flow of the headbox, the consistency of the headbox,retention, drainage, wire speed, low-pressure levels, driving output andthe power used for providing vacuum. The multivariable controller is ahigh level controller that calculates new set values for localcontrollers of a lower level based on the measurements.

FIG. 3 shows a preferable way to optimise the operation of the wiresection. According to the solution, feedback is taken into account incontrol both for the paper quality using formation and for the processby considering how the consistency has developed on the wire section 2.The form ation can be determined, for instance, using the secondmeasuring instrument 15 by measuring the optical on line formation afterthe dryer section 5. When measuring the formation using a measuringinstrument that moves back and forth in the transverse direction of thepaper web 3, the formation can be considered particularly on such alocation of the paper web 3, which is discovered to react susceptibly onthe changes regarding drainage. For example, a starting sheet crushingmay appear as a stripe at a particular place on the paper web 3. How theconsistency develops on the wire section can also be determined bymeasuring it locally. Implementing the consistency measuring is,however, a demanding task, and therefore a calculatorily determinedconsistency can be utilized. In principle, the development of theconsistency can be determined, when the consistency of the headbox isknown as well as the water amount drained by each dewatering element inthe wire section. The consistency calculated in this way is, however,inaccurate. What causes inaccuracy in this method is, for instance, thefact that most of the drainage of the wire section 2 takes place in thearea of the forming roll 10. Thus, the consistency is most preferablydetermined using backward calculation. The basis is then the dry solidscontent of the paper web 3 after the press section 4 and the basisweight after the dryer section 5. Such data can be obtained, forexample, using measuring instruments 14 and 15. The amount of waterdrained on the press section 4 can be measured, whereby the dry solidscontent can be determined after the wire section 2. How the consistencyis developed on the wire section 2 can be determined by means ofbackward calculation, when the amount of water drained by the dewateringelements in the wire section 2 is known, i.e. a model is developed onhow the consistency develops on the wire section 2.

Since the dependencies between different properties are not linear andthe process includes several interference factors, which are taken intoaccount preferably by implementing the adjustment using a fuzzycontroller. For example, owing to the variety of stock properties, it isdifficult to definitely predict the magnitude of the consistencyarriving at the dewatering element in order to provide the best possibleformation. Therefore, the control must be provided with feedback to thepaper quality. When fuzzy control is concerned, the restricting factorssuch as maximum slice flow, required dry solids content succeeding thewire section or required formation/porosity can be considered as theboundary conditions. Owing to the boundary conditions, the dewateringelement cannot be provided for instance with a vacuum that is too highas a set value. The two-sidedness of the fines and filler distributionobtained during laboratory measuring can be taken into account when anoptimal distribution is calculated for drainage between the upper andlower sides of the web on the wire section 2. This can be consideredwhen the low-pressure settings of the first dewatering elements or thewire tensions of the wire section are concerned, whereby the drainagecan be controlled more accurately in the upper or lower direction of theweb. If the tensions of the wires are not adjusted, then the excessdrainage at the upper or lower sides can be prevented by providing thecontroller with boundary conditions, within the scope of which thelow-pressure settings and thus the dewatering distribution can bechanged. Furthermore, the database of the boundary conditions mayinclude data on, for instance, the allowed consistency after themultifoil shoe or on the consistency after the suction box. The databaseof the boundary conditions may further include the data about the flowconcerning a short circulation.

The element-specific drainage measurements to be carried out allowcalculating the dewatering distribution between different elements. Anoptimal dewatering distribution can be found by optimising the drainageof the elements heavily affecting the formation of the bottom, i.e. theforming roll 10 and the multifoil shoe 11, to such a point, in which theformation and/or porosity measurement provides a set value.

The operation of the wire section 2 is optimally adjusted for instancein accordance with the principle shown in FIG. 3. The effect of theconsistency on the formation of the web must be determined foroptimisation. It has been noted in performed tests that when theconsistency arriving at a particular dewatering element decreases, theformation improves, but only to a certain extent. When the consistencydecreases too much, the formation becomes drastically worse, as thealready infiltrated fibre matting is damaged, when being loaded withpressure pulses, while the consistency is still too low. This so-calledsheet crushing evidently causes detectable stripes to the paper web 3. Acurve shaped as curve A in FIG. 3 depicts the value of the form ationwhile the consistency changes. The shape of curve B corresponds to theshape of curve A and both curves A and B describe in FIG. 3 theoperating areas of the wire section.

At first, the operating area has to be determined, i.e. whether theoperating area is curve A or curve B or another substantially, equallyshaped curve placed somewhere else on the horizontal axis. Basic data onthe structure of the paper machine and on the stock is required fordetermining the operating area. The operating area is determined byidentifying the papermaking process within the area of the wire section.A dynamic model is developed from the process that is continuouslyspecified using the measurements of the process. The shape of curves Aand B is therefore determined experimentally.

When the operating area, for instance A or B, is determined, theoperation of the wire section is to be adjusted so that an operatingpoint, i.e. x₀, y₀, providing the best possible formation is formed asthe operating point. The operating point x₀, y₀ should therefore be asclose as possible to the zenith of curve A or B. If the operating pointx₀, y₀ is exactly at the zenith, there is a risk that the formationabruptly starts to decrease, when the starting values of the processchange, if the consistency is reduced too much. The operating point x₀,y₀ should therefore be selected slightly to the right of the zenith.However, in the solution of the invention, the operating point x₀, y₀need not be moved far away from the zenith, since the operation of thewire section can be perfectly controlled. It should also be considered,when determining the operating point x₀, y₀, that the paper web 3 mustnot be too wet after the wire section 2. In other words it should benoted when determining the operating point that the dewatering capacityof the end part of the wire section is sufficient in order to provide adry solids content that is adequately high.

The shape of curves A and B are thus known, meaning that they are knownin the following formy−y ₀ =f(x−x ₀), where

-   -   y is formation,    -   y₀ is formation at a reference point,    -   x is consistency and    -   x₀ is consistency at the reference point.

An optimal value must therefore be determined for the operating pointx₀, y₀. If the shape of curve A or B is flat, meaning that the formationdoes not decrease too rapidly below a certain consistency, the operatingpoint x₀, y₀ can be determined utilizing a first derivative. Whendetermining the operating point, the property, i.e. consistency, on thex-axis is systematically disturbed, and variable dy/dx is at the sametime continuously determined, i.e. the ratio between the firstderivative of the formation and the first derivative of the consistency,i.e. d(formation)/d(consistency). Disturbing the consistency means thatthe consistency is changed for example by changing the vacuum of adewatering unit. The disturbance takes place systematically in such amanner that minor changes are continuously caused to the consistency.The variance in the consistency must naturally be adequately large,meaning that the consistency change should cause a change in theresponse, i.e. in the form ation. The variabled(formation)/d(consistency) thus refers to the formation change afterthe change in the consistency has taken place. The variable dy/dx can bedetermined constantly or at appropriate intervals.

In an optimisation situation the consistency is controlled in such adirection that the derivative or dy/dx becomes the desired, i.e. thevalue thereof is close enough to zero. In this way the operating pointis search for on curve A or B, where the slope of the curve isappropriate when the operating point is located on the right from thetop of the curve. A threshold value can fairly easily be determined byexperimenting when the size of the derivative is adequately close tozero

When an operating point is optimised, formation, porosity and dry solidscontent can be selected as adjustable properties, and headbox sliceflow, vacuum of a forming roll and vacuum of a multifoil shoe can beselected as adjusting properties or variables. In such a case, threeadjustable properties and three adjusting variables are provided,whereby for instance simplex optimisation can be used in optimisation.

If the shape of curves A or B is inconvenient, i.e. when the consistencychanges the formation drastically drops, an optimal operating point x₀,y₀ can also be found in such a case, but then a second derivative of theformation is also utilized. In such a case, the ratio between the firstderivative of the formation and the first derivative of the consistencyis determined and thereafter the ratio between the second derivative ofthe formation and the first derivative of the consistency is determined.The shape of the curve in the operating point x₀, y₀ is determined basedon the second derivative. A control step size of the consistency isdetermined based on the shape of the curve. If the second derivative isvery high, it indicates that the direction of the curve changes veryrapidly, whereby the control step size must be very small. Anappropriate threshold value of the size of the second derivative and thelength of the control step to be determined based on the secondderivative can be determined fairly easily by experimenting. The controlstep must be so small that the operating point is not driven to anundesired operating area, meaning that the formation should notdeteriorate extensively. The dead time of the process is obviously takeninto account between the control steps, said dead time determining howoften control steps can be provided, i.e. how often the consistency canbe changed. In addition, if the second derivative indicates that thedirection of the curve changes very rapidly, the operating point shouldnot be optimised close to the top, so as not to be placed on the leftside of the top in an undesired operating area starting shortly. If, inturn, the second derivative indicates that the curve is very flat, it ispossible to place the operating point close to the top. In brief, thevariable d(formation)/d(consistency) allows determining the correctcontrol direction of the consistency and the size of the control stepcan again be determined using the variabled(d(formation)/d(consistency).

If the consistency has to be changed, the boundary conditions will bechecked. If the boundary conditions prevent the consistency to bealtered, then no changes can be made, instead the process returns todetermine the consistency and the formation. If the boundary conditionsallow making changes, such as controlling the slice opening 1 a of theheadbox 1 or adding or reducing the vacuums of the dewatering elements,then the control method shown allows optimising the drainage state,whereby it is possible on the wire section to divide the drainagebetween different elements in such a manner that the best possibleresult is obtained regarding the formation, porosity, distribution ofthe upper and lower sides of drainage and the final dry solids content.

It should be noted when the operation of the wire section is adjustedthat in the process the wire section is provided with two velocityconstants as far as consistency is concerned. Firstly, the consistencymay change rapidly when the properties of the stock change. Disturbance,such as disturbance in the basis weight, in the stock may thereforecause a rapid change to the consistency. Then again, when the optimumvalue of consistency is searched for, the consistency changes fairlyslowly. Consequently, the shape of curves A and B shown in FIG. 3changes very slowly.

The porosity and formation react differently to different changes. Ifthe stock becomes more compact, the porosity decreases. However, it hasbeen observed when adjusting the wire section that if the properties ofthe stock remain unchanged and the wire section is simultaneouslyadjusted in such a manner that the formation increases, the porosityincreases at the same time. Consequently, the porosity may set aboundary condition on how to adjust the wire section in order to improvethe formation. Instead of aiming to improve the formation, the wiresection can also be adjusted in such a manner that the aim is to providesuch a porosity that is as appropriate as possible for the finalproduct. The adjustment based on porosity can principally be carried outin a corresponding manner as is shown in FIG. 3. Thus, the formationnaturally may set a boundary condition on how to adjust the porosity.

Instead of determining the ratio between the derivative of the formationand the derivative of the consistency in FIG. 3, a ratio between thederivative of the porosity and the derivative of the consistency can bedetermined. In addition, the quality property of paper can be determinedcomprising the formation and/or the porosity and/or a combinationdefined by means of a formation and/or porosity. Also, in addition to orinstead of the above possibilities, a cost function can be determinedthat includes at least the effect of the formation and/or the porosityand may further comprise, for instance, the effect of a wire sectionand/or the dewatering elements thereof on the consistency, and theconsistency developed on the wire section is adjusted by optimising thecost function. Thus, when determining the operating point and searchingfor the correct control direction of the consistency, a ratio betweenthe first derivative of the quality property of paper and/or the costfunction and the first derivative of the consistency is determined, andwhen determining the size of the control step, a ratio between a secondderivative of the quality property of paper and/or the cost function andthe first derivative of the consistency can be determined.

When changing the grade, grade-specific set values stored in thedatabase of the control system can be used as rough set values; thegrade-specific set values are introduced in accordance withpredetermined ramping. The grade change does not lead to an unstableoperating area and the optimal running values can be rapidly achievedafter the grade change.

FIG. 4 shows a solution, how the vacuum in the dewatering elements atthe end part of the wire section can be optimally distributed betweendifferent elements. At this stage, the consistency is so extensive thatthe of the paper web, such as formation or filler distribution, cannotbe affected any longer. This occurs when the consistency is so high thatthe fibres can no longer move in relation to one another. In general,this occurs within the consistency range from 6 to 10%. What isdetermined from the paper is the dryness development and from theprocess the vacuums of the elements and the paper machine speed. What isalso known is the structure geometry of the wire section, whereby theeffective suction time in each dewatering element can be determined.Curves C1 to C4 indicated with a dashed line depict the drainage abilityin each dewatering element. An unbroken curve describes how the drysolids content is developed in the paper web as a function of theaffecting time of vacuum. Curves C1 to C4 can be determined for instancein accordance with publication Räisänen K., “Water Removal by Flat Boxesand a Couch Roll on a Paper Machine Wire Section”, dissertation,University of Technology, Paper Technology Laboratory, Espoo, 1998, page62. Curves C1 to C4 may depict the drainage of successive couch rollsand/or suction boxes. Furthermore, the cost functions are taken intoaccount regarding the wear of the wires and the energy consumption ofthe low-pressure pumps. The database of the boundary conditions includesdefined values for the consistency succeeding the wire section andboundary conditions for vacuums. Based on these values the optimal valueis determined, whereby the computational model allows distributing thevacuum between different elements in such a manner that the need forsuction energy is as small as possible and the wear of the wires remainsminimal. What is obtained by means of on line process measurements andconsistency calculation is continuous feedback from the dryness, andthus models describing the development of the dryness functioning as thebasis of the optimisation calculation can be continuously updated.

Because the wire wears, the vacuums must be changed in order to obtain adesired consistency level on a particular location of the former. Inaddition, the development of the consistency must be changed, if thevacuums are too high, in which case the wire wears rapidly. Theconsistency is preferably adjusted by optimising on the basis of such acost function that includes a quality deviation cost and a control cost.The quality deviation cost then takes into account the runnability ofthe process and/or a dry stuff requirement set by the quality propertyof paper between different elements and/or the wire section and thepress section, and the control cost, in turn, observes the requiredpower and/or the driving output of the wire section in order to achievedrainage.

The drawings and the specification associated therewith are merelyintended to illustrate the idea of the invention. The details of theinvention may vary within the scope of the claims. Thus, another type offormer solution than the gap former may form the wire section, and inaddition to or instead of the dewatering elements shown in FIG. 1 otherdewatering elements can be used as the dewatering elements of the wiresection. Other methods than vacuum and tensions of the wire can also beused to provide the web with a dewatering pressure.

1. A method for adjusting the operation of a wire section, the methodcomprising determining the development of the consistency of stock on awire section, determining the effect of the consistency determined aboveon the formation and/or porosity of a paper web and adjusting theconsistency developing on the wire section based on a quality propertyof paper and/or by optimising a cost function, the quality property ofpaper comprising the formation and/or the porosity and/or a combinationdefined by means of the formation and/or the porosity, and the costfunction including at least the effect of the formation and/or theporosity.
 2. A method as claimed in claim 1, wherein the cost functioncomprises the effect of the wire section and/or the drainage ability ofthe dewatering elements thereof on the consistency.
 3. A method asclaimed in claim 1, the method comprising determining an optimum valuefor the consistency in such a manner that a ratio between a firstderivative of the quality property of paper and/or the cost function anda first derivative of the consistency is determined.
 4. A method asclaimed in claim 3, the method comprising searching an optimal operatingpoint for the process regarding the quality property of paper bydetermining a correct control direction of the consistency bydetermining the ratio between the first derivative of the qualityproperty of paper and/or the cost function and the first derivative ofthe consistency and determining the size of a control step of theconsistency by determining a ratio between a second derivative of thecost function and/or the quality property of paper and the firstderivative of the consistency.
 5. A method as claimed in claim 1, themethod comprising determining the development of the consistency on thewire section by determining the dry solids content in the paper webafter the wire section and measuring the amount of water drained by thedewatering elements of the wire section and calculating the developmentof the consistency on the wire section based on the drained amount ofwater.
 6. A method as claimed in claim 1, the method comprisingdetermining the development of the consistency on the wire section bymeasuring.
 7. A method as claimed in claim 1, wherein the consistency isadjusted by regulating a slice opening of a headbox and/or the drainageof a dewatering element.
 8. A method as claimed in claim 1, wherein theconsistency is adjusted by optimising on the basis of such a costfunction that includes a quality deviation cost and a control cost,whereby the quality deviation cost takes into account the runnability ofthe process and/or a dry stuff requirement set by the quality propertyof paper between different elements and/or the wire section and thepress section, and the control cost observes the required power and/orthe driving output of the wire section in order to achieve drainage. 9.A method as claimed in claim 1, wherein the consistency is optimallyadjusted regarding the paper quality utilizing on line measurementsconcerning the paper quality.
 10. A method as claimed in claim 1,wherein a dynamic model is formed concerning the development of theconsistency on the wire section, the dynamic model being updated on thebasis of the measurements.
 11. A method as claimed in claim 1, wherein afuzzy controller is utilized for adjusting the consistency, in whichboundary conditions are determined for the adjustable consistency andfor the adjusting variables.
 12. A method as claimed in claim 1, themethod comprising determining the drainage ability of the dewateringelements in the end portion of the wire section as a function of timeand based on said definition the dry solids content of the web providedby the wire section is optimally adjusted by adjusting the drainageability of the dewatering elements.
 13. A method as claimed in claim 12,the method comprising forming a dynamic model describing the developmentof dry stuff, and updating the model based on the measurements anddeciding upon the optimal drainage of the wire section in accordancewith said model.
 14. An apparatus for adjusting the operation of a wiresection, the apparatus comprising means for determining the developmentof the consistency of stock on a wire section, means for determining theeffect of the consistency determined above on the formation and/orporosity of a paper web and means for adjusting the consistencydeveloping on the wire section based on a quality property of paperand/or by optimising a cost function, the quality property of papercomprising the formation and/or the porosity and/or a combinationdefined by means of the formation and/or the porosity, and the costfunction including at least the effect of the formation and/or theporosity.
 15. An apparatus as claimed in claim 14, wherein the costfunction includes the effect of the wire section and/or the drainageability of the dewatering elements thereof on the consistency.
 16. Anapparatus as claimed in claim 14, the apparatus comprising means fordetermining an optimum value for the consistency by determining a ratiobetween a first derivative of the quality property of paper and/or thecost function and a first derivative of the consistency.
 17. Anapparatus as claimed in claim 16, the apparatus comprising means fordetermining a correct control direction of the consistency bydetermining the ratio between the first derivative of the qualityproperty of paper and/or the cost function and the first derivative ofthe consistency and determining the size of a control step of theconsistency by determining the ratio between a second derivative of thecost function and/or the quality property of paper and the firstderivative of the consistency.
 18. An apparatus as claimed in claim 14,the apparatus comprising means for determining the dry solids content inthe paper web after the wire section and measuring means for measuringthe amount of water drained by the dewatering elements of the wiresection and calculation means arranged to calculate the development ofthe consistency on the wire section based on the drained amount ofwater.
 19. An apparatus as claimed in claim 14, the apparatus comprisingmeans arranged to measure the development of the consistency on the wiresection.
 20. An apparatus as claimed in claim 14, wherein the means foradjusting the consistency comprise means for adjusting the drainage inthe dewatering elements of the wire section and/or means for adjusting aslice opening of a headbox.
 21. An apparatus as claimed in claim 14,wherein the adjustment apparatus for adjusting the consistency is amultivariable controller.
 22. An apparatus as claimed in claim 21,wherein the multivariable controller utilizes a dynamic model describingthe development of the consistency on the wire section, and the model isarranged to be updated on the basis of the measurements.
 23. Anapparatus as claimed in claim 14, the apparatus comprising a fuzzycontroller, in which boundary conditions are determined for theadjustable consistency and for the adjusting variables.